Transcript EQ I - Facts, Rebound, & Seismograph

EARTHQUAKES
MOTHER
NATURE’S
RUMBLINGS
What is that force?
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Earthquake – a vibration of the solid
earth produced by the very rapid
release of energy
The energy that drives these quakes
is derived from the earth’s interior
The motions of the earth’s plates are
frequently the cause of this rapid
energy release
Why bother in the first place?
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Why do we study
these earthquakes?
• Pure scientific
curiosity
• Try and learn how to
predict them
• Gives us a view of
the earth’s interior
• So we can try and
avoid tragedies like
the one shown here
In 1985, an earthquake
measuring over 8.0 struck
outside of Mexico City
An estimated 25,000 people
were killed in this tragedy
Where in the world are they?
This map show world earthquake distribution
What familiar pattern do these earthquake locations line up with?
That’s right, the tectonic plates!
Where do they happen?
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Focus – The point of
origin of an Earthquake
The earth’s lithosphere
ruptures, and energy
radiates out in all
directions
Most earthquakes do
not occur directly at
the surface
Epicenter – a location
on the surface directly
above the focus
A diagram showing a fault
with an earthquake focus
occurring at depth with the
epicenter directly above it
How deep?
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Although it’s possible, most
earthquakes do not happen
right at the earth’s surface
The depth that they occur,
depends upon the plate
boundary
Convergent boundaries
produce the largest quakes
and also the deepest ones—
600 km depth is maximum
Transform boundary quakes
can also be large but are
generally shallow—less than
80 km below the surface
Divergent boundaries
produce weaker, shallower
quakes as well
This map and cross section of the
Japanese Island Arc shows the
range of earthquake depths found
at convergent subduction zones
Waves on the Water
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Just as waves radiate out
from a stone being
dropped in water, so will
earthquake waves radiate
out in all directions
EQ waves will radiate out
from the focus in 3-D
Energy dissipates very
rapidly through solid rock,
but sensitive instruments
can pick up, that is “listen
for” seismic events all
around the globe
Whose fault is it?
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Most earthquakes occur
along faults or fault
zones
Fault – a fracture in the
earth’s crust along which
there is movement
Some are a few km in
length, others 1000s of
km long
Faults come in all
varieties of angles,
depths, and shapes
Earthquake Processes
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No one knew how earthquakes worked
until early in the 20th century
H. F. Reid studied the 1906 San Fran
quake and came up with a theory called
elastic rebound
Just like a stick
under pressure,
rocks will bend
to a certain point
and will then
rupture and
release stored
energy with
violent shaking
Understand the Physics
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Elastic rebound can be
explained once the idea
of stress and strain is
understood
Stress = pressure
Strain = deformation
Rocks in the crust are
under incredible
amounts of pressure
and will slowly deform
Once the rocks’
strength is exceeded
they will rupture causing
vibrations
There are different kinds of
stress as demonstrated by the
above diagram
The Theory Behind the Shake
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Elastic Rebound –
sudden released of
stored strain in
rocks
The result of this
rebound is sudden
movement along a
fault
The rupture of this fault trace in
California was formed due to
elastic rebound
The strength of the rocks could
only hold so much stress and then
they gave way
What comes before and after?
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Foreshocks – small earthquakes
preceding a major quake by days or
even possibly years
Aftershocks – the settling or
adjustment of crustal rocks after a
large earthquake
Seismology
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The entire branch of
geology dedicated to
studying earthquakes
and earthquake waves
is called seismology
Seismographs – the
instruments used to
measure and record
seismic waves
Almost 2000 years ago
the Chinese made the
very first seismograph
The original seismograph was invented
by the Chinese 2000 years ago
Metal spheres were delicately balanced
in mouths of dragons, and the slightest
vibrations would cause them to fall out
Modern Seismograph
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Modern seismographs
incorporate the
principle of inertia
Inertia – an object at
rest will stay at rest, &
an object in motion will
stay in motion
Seismographs are
placed at depth away
from background noise
Always in a group of
three to measure all
three dimensions of
movement (3-D)
A modern seismograph has a pen attached to a
weight on a spring; the ground shakes the
rotating drum while the pen remains stationary
due to inertia and traces out the quake
Look at the Data
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When seismographs
record earthquake
activity onto paper it
is called a seismogram
Top: simple seismogram
showing the arrival of
different P & S waves
Bottom: a long track
seismogram showing a
major earthquake
accompanied by both a
foreshock and an
aftershock
To Review:
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Earthquakes are the result
of a rapid release of stored
energy from within the crust
The focus is where the
EQ occurs and the
epicenter is on the surface
directly above the focus
EQ happen along faults
which are long cracks in the
earth’s crust
Earthquake energy
radiates out in all directions
like ripples on a pond
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H. F. Reid came up with
theory of elastic rebound
when studying the great
1906 quake of San Fran
This theory is used to
explain EQ even today
Stress (pressure) causes
strain (deformation)
Seismograph is the
instrument used to measure
EQ and it operates on the
law of inertia
Seismograms are recorded
earthquakes on paper